Compositions formed of mixtures of asphalt and aggregates materials are one of the most widely used materials for paving roads and highways. Although the term "asphalt" is commonly used to refer to the road material, it more properly applies to the "tar" portion of a mixed composition of tar and aggregate. Thus, the asphalt is a dark brown or black cementitious material, which is solid or semi-solid in consistency, in which the predominating constituents are bitumens which occur in nature or are obtained as byproducts from petroleum refining. Asphalt is a mixture of paraffinic and aromatic hydrocarbons and heterocyclic compounds containing sulfur, nitrogen and oxygen. Asphalt is also referred to as "petroleum asphalt," "Trinidad pitch," or "mineral pitch." Asphalt is a black solid or viscous liquid that has a flash point of about 450.degree. F. (232.degree. C.), an auto-ignition temperature of about 900.degree. F. (482.degree. C.), and softens from its solid or semi-solid state to a viscous liquid at approximately 93.degree. C. Other typical uses of asphalt include roofing, joint filling, special paints, adhesive and electrical laminates, and hot belt compositions, a dilutent in low-grade rubber products and a number of other applications. These and other properties and uses of asphalt are generally well known in the art and can be found, for example, in Lewis, Hawley's Condensed Chemical Dictionary, 12th Edition (1993).
In paving compositions, asphalt is typically present in an amount of about 5% by weight. With respect to such compositions, the percentage of asphalt and the nature and size of the aggregate material (typically rocks and sand) used to make the composition are important for the proper structure and characteristics of the final road structure. For example, typical roads are formed of three layers of asphalt and aggregate compositions. The bottom most layer includes rocks of one inch or greater average size, sand, and the asphalt. An intermediate layer generally includes a composition formed of somewhat smaller rocks, typically 0.5-1 inch in diameter, again with sand and asphalt. Finally, a top layer is usually applied which has the smallest rocks, typically 0.5 inches or less in diameter plus sand and tar.
Because the aggregate generally represents more than 90% of a hot asphalt mix, aggregate gradation (i.e., the different particle sizes that are present in the blend) profoundly influences the properties of the hot mix (such as air voids, workability, and the amount of asphalt binder required) and the resulting properties of the pavement (such as stiffness, stability, and durability) (e.g., Aljassar et al., Toward Automating Size-Gradation Analysis of Mineral Aggregate, Transportation Research Record, Issue Number: 1437, pp. 35-42 (1994)). In this regard, research on asphalt-aggregate compositions has become quite detailed, including for example quantifying the influence on resistance to rutting when rounded, smooth, sand-sized aggregate particles are replaced by rough, angular, porous particles while other aggregates and the total gradation remain unchanged (e.g., Perdomo, D. and Button, J. W., Identifying and Correcting Rut-Susceptible Asphalt Mixtures, Final Report, Texas Transportation Institute, Texas A&M University, Texas State Department of Highways & Public Transp, Federal Highway Administration, Report Number: Fhwa/Tx-91/1121-2F;Res Rept 1121-2F;TTI: 2-8-87/91-1121, Pag: 164p), or evaluating the effect of the amount of soil binder (i.e., the smallest aggregate particles) on the engineering properties of asphalt-treated paving materials (Ping, and Kennedy, The Effects of Soil Binder and Moisture on Black Base Mixtures, Texas University, Center for Highway Research, Austin, Texas State Department of Highways & Public Transp Report Number: FHWA/TX79/08+183-12 Intrm Rpt.;FCP 45C2-352 Pag: 127p (1997)). Aggregate gradation is frequently determined by the well-known and widely used sieve analysis method.
Accordingly, as these exemplary references indicate, depending upon the conditions under which a road is used (e.g., traffic patterns and weather conditions), the composition of any one or more of the asphalt layers must be carefully designed and monitored. Additionally, because the aggregate materials are typically taken from local quarries, and the manufacture of the composition is not an exact science, the asphalt and aggregate compositions must be frequently tested, both as they are being made and after they have been applied to a roadway, to make sure that they meet the appropriate requirements.
Thus, there exists a need for determining: (1) the weight percentage of asphalt; and (2) the aggregate size distribution in a given sample of an asphalt-aggregate composition. In one conventional method of analysis, the percentage of asphalt is determined by a solvent extraction technique which uses chlorinated hydrocarbons to separate the asphalt from the aggregate materials. Because the solvents are generally considered to raise a hazard to persons who are exposed to their vapors, the solvent testing is becoming more and more disfavored, and indeed is expected to eventually become prohibited under appropriate environmental regulation.
In a second conventional method, a weighed sample of the composition is placed into a furnace which is then heated until the asphalt in the composition ignites. The asphalt is then allowed to burn until it is entirely consumed after which the remaining aggregate is weighed. The difference between the starting and ending weight is a measure of the composition of asphalt in the composition.
There are, however, at least two problems with this conventional ignition technique. First, in conventional conduction and convection heating, the burning asphalt can carry the combustion to temperatures of up to 900.degree. C., i.e., a state which is somewhat out of control. These extreme temperatures can cause particular problems in the equipment or in handling the hot material or even confining the resulting fire.
Second, and perhaps just as important, the extreme temperatures tend to degrade the physical characteristics of the aggregate in the mixture, including its size. Thus, because the aggregate remaining after the asphalt has been burned off is typically measured to determine whether it is of the proper size, the size degradation resulting from the conventional ignition test leads to a certain inaccuracy in measuring the sizes of aggregate in any given sample.
As another problem, if the aggregate contains carbonate compounds, the excessive heat can drive off carbon dioxide, thus changing both the chemical and physical characteristics of the aggregate. Finally, the conventional ignition techniques tend to cause a loss of "fine" aggregates--literally blowing them away--so that such fines are neither sized nor weighed, further acerbating the accuracy problem.
Accordingly, the need exists for a method of determining the amount of asphalt in an asphalt-aggregate combination which avoids the use of environmentally disfavored solvents, which more carefully controls the combustion and which avoids the breakdown in the aggregate materials that tends to result in improper sizing following such testing.
In co-pending parent application Ser. No. 09/045,392 an improved technique for asphalt-aggregate analysis is disclosed that takes advantage of the properties of microwave radiation to provide improved methods and apparatus for the combustion-based analysis of asphalt-aggregate mixtures. It has now been discovered that such apparatus and methods can be enhanced--and the time required for analysis greatly reduced--by additional control of the appropriate airflows, and by incorporating improved structural elements.